1. Field of the Invention
This invention relates to an apparatus and method for reducing compressive forces on soft tissue disposed between a bone, such as a human pelvis, and a supporting structure, such as the seat of a wheel chair.
2. Description of the Related Art
Many immobile (i.e. paraplegic, quadriplegic, cerebral palsy) patients are seated in a wheelchair during all or most of their waking hours. Open skin wounds or sores (decubitus ulcers) form on the buttocks of the patient due to a lack of blood flow in the skin from the constant pressure of their own weight. The ischial tuberosity is the most common site for pressure sores, accounting for 28% of all ulcers. Pressure sores are a significant and common cause of morbidity and mortality in spinal cord injury patients. Considering the gradual decline in mortality rates from spinal cord injuries, coupled with an aging population, it is conceivable that the group of patients prone to developing pressure sores will increase in the future. The bulk of the literature to date has primarily focused on the technical aspects of surgical management of pressure sores.
It is well established that the principle, and often solitary, cause of the decubital ulcers is excessive pressure, usually on a bony prominence in susceptible individuals. Sitting posture naturally creates high contact pressures at both ischial tuberosities, the coccyx, and in some cases the greater trochanters. The magnitude of contact pressure over ischial tuberosities has been found to correlate well with incidence of pressure sores. As a result of immobility and impaired protective sensation, wheelchair-bound spinal cord injury patients are at ongoing significant risk of pressure sores in these regions.
The needs of immobile patients in a wheelchair and their caregivers vary widely. The physical functionality of spinal cord injury patients has wide variation based on the position of the injury along the spinal cord. Patients with cerebral palsy and other degenerative diseases each have their own set of wheelchair requirements. Another important consideration is that most people confined to a wheelchair commonly have chronic shoulder and elbow injuries. Caregivers also suffer from chronic problems. Transferring patients in and out of a wheelchair and the weight of the wheelchair itself contribute to these chronic problems. As blood flow in the skin under compression below the lower pelvis in patients confined to wheelchairs becomes better understood, the needs of patients and caregivers will be further delineated.
Skin pressure can be reduced by partial levitation of the patient using repulsive forces generated by magnets. Permanent magnets may be surgically implanted in the patient's lower pelvis (ischium) and magnets below the seat or at the seat surface of a wheelchair repel the magnets surgically implanted in the lower pelvis. The purpose of these magnets is to partially lift the patient. Partial levitation of the patient allows blood to flow in the patient's skin, thereby reducing the tendency for decubitus ulceration. See, for example, U.S. Pat. No. 6,895,973 and corresponding U.S. Patent Application Publication No. 2004/0077922.
However, there is still a need for an improved magnetic levitation system that provides for enhanced lift and load shift capabilities for immobile patients in a wheelchair.